The present invention relates to machine tools, and more especially those designed to drill holes at very high speeds of rotation of the tool (even exceeding 100,000 revolutions per minute).
More specifically, the invention is concerned with machine tools in which, to prevent friction between the spindle casing and the rotating parts fixed to the tool, aerostatic bushes, known to those skilled in the art, are interposed between the two. In aerostatic bushes the rotating parts are supported by cushions of air generated in the air gaps by the continuous feeding in of a jet of air which, after having travelled through the said air gaps performing its bearing function as described, passes out of the spindle through the same aperture formed in a flange mounted on the free end of the spindle, as that through which the tool-holding collet projects.
In the high-speed drilling work mentioned above, and especially where the holes are being made in materials of low specific weight such as the resins for printed circuit boards, a fine dust is thrown up, consisting of the minute chips that are generated, which is deposited in the radial slits formed in the end of the tool-holding collet to enable it to be opened and closed around a tool. This necessitates cyclical cleaning and maintenance operations, which, in view of the large total number of spindles usually used in a plant and the frequency of the drilling operations, represents a significant addition to production costs. The reason for this is that although the aforementioned jet of air passes out through the said aperture parallel to the tool-holding collet, it is then deflected by vortices generated by the very high-speed movement of the edges and sides of the collet and begins to move in a direction which has a large radial component leading away from the said collet. It cannot therefore exert any action that would keep the said fine dust away from the abovementioned radial slits.
In other words, in front of the end of the collet, a zone is generated in which the air is effectively stagnant, allowing the granules of the said fine dust to pass through it and reach and be deposited in the abovementioned slits.